This proposal is about the macaque monkey's striate cortex: determining how it represents spatial information though orientation tuning and 2-D spatial interactions by measuring the dynamics of cortical neurons and neural networks with single cell electrophysiology. Orientation tuning dynamics. We need to extend our preliminary measurements of orientation dynamics: First, to use smaller and larger stimulus areas to see how much of the dynamics is due to local circuit interactions within hypercolums, and how much to long-range connections; Second to calculate to what extent a stimulus in the past nonlinearly modulates the response, and how such nonlinearities affect orientation tuning. Contrast. I will measure the effect of contrast on the observed orientation dynamics since we know that V1 responses are slowed at low contrast. Measurements at lower contrast will enable us to resolve better the processes that affect the time evolution of orientation tuning. Apertures Feedforward models imply that orientation tuning should degrade when the test patterns are viewed through very small apertures, but feedback models can retain their tuning. Based on this idea, measuring orientation tuning through different-sized apertures is a high priority. The key here will be to use apertures on the spatial scale of receptive field subregions (<1 degree), to test how much local spatial summation accounts for orientation tuning. Receptive fields We will use the subspace reverse correlation method to characterize the 2-D spatiotemporal sensitivity of V1 neurons, and to study contrast effects on the 2-D spatiotemporal receptive fields. Contest, and boundary completion. Long distance effects on cortical responses, due to image context effects such as boundary completion and figure/ground segregation, may be significant in V1, but they may be encoded in the temporal pattern of the response rather than just in the spike count. In single cells and visual evoked potentials (VEPs), we will measure the spatiotemporal dynamics of response to a patch in the middle of the receptive field in the presence of different kinds of backgrounds: spatially uniform, black, or a spatial extension of the pattern in the central patch. I will be seeking a change in the dynamics of the response the is tied to the figure/ground organization of the scene. We also will search for context effects embedded in 2nd order responses.